# std::flat_multimap

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Defined in header `<flat_map>` |  |
template< class Key, class T, class Compare = std::less<Key>, class KeyContainer = std::vector<Key>, class MappedContainer = std::vector<T> > class flat_multimap; |  | (since C++23)

The flat multimap is a container adaptor that gives the functionality of an associative container that contains key-value pairs, while permitting multiple entries with the same key value. Keys are sorted by using the comparison function `Compare`.

The class template `flat_multimap` acts as a wrapper to the two underlying containers, passed as objects of type `KeyContainer` and `MappedContainer` respectively. The first container is sorted, and for each key its corresponding value is in the second container at the same index (offset). The number of elements in both containers is the same.

Everywhere the standard library uses the Compare requirements, uniqueness is determined by using the equivalence relation. Informally, two objects a and b are considered equivalent if neither compares less than the other: !comp(a, b) && !comp(b, a).

`std::flat_multimap` meets the requirements of Container, ReversibleContainer, optional container requirements, and all requirements of AssociativeContainer (including logarithmic search complexity), except that:

- requirements related to nodes are not applicable,
- iterator invalidation requirements differ,
- the complexity of insertion and erasure operations is linear.

A flat multimap supports most of AssociativeContainer's operations that use equal keys.

### Iterator invalidation

| This section is incomplete

### Template parameters

Key | - | The type of the keys. The program is ill-formed if `Key` is not the same type as `KeyContainer::value_type`.
T | - | The type of mapped values. The program is ill-formed if `T` is not the same type as `MappedContainer::value_type`.
Compare | - | A Compare type providing a strict weak ordering.
KeyContainer MappedContainer | - | The types of the underlying SequenceContainer to store keys and mapped values correspondingly. The iterators of such containers should satisfy LegacyRandomAccessIterator or model `random_access_iterator`. Invocations of their member functions `size` and `max_size` should not exit via an exception. The standard containers std::vector and std::deque satisfy these requirements.

### Member types

Type | Definition
`key_container_type` | `KeyContainer`
`mapped_container_type` | `MappedContainer`
`key_type` | `Key`
`mapped_type` | `T`
`value_type` | std::pair<key_type, mapped_type>
`key_compare` | `Compare`
`reference` | std::pair<const key_type&, mapped_type&>
`const_reference` | std::pair<const key_type&, const mapped_type&>
`size_type` | std::size_t
`difference_type` | std::ptrdiff_t
`iterator` | implementation-defined LegacyInputIterator and `random_access_iterator` to `value_type`
`const_iterator` | implementation-defined LegacyInputIterator and `random_access_iterator` to const value_type
`reverse_iterator` | std::reverse_iterator<iterator>
`const_reverse_iterator` | std::reverse_iterator<const_iterator>
`containers` | type describing the underlying containers struct containers { key_container_type keys; mapped_container_type values; };

### Member classes

value_compare | compares objects of type `value_type` (class)

### Member objects

Member | Description
`containers` `c` (private) | the adapted containers (exposition-only member object*)
`key_compare` `compare` (private) | the comparison function object (exposition-only member object*)

### Member functions

(constructor) | constructs the `flat_multimap` (public member function)
(destructor) (implicitly declared) | destroys every element of the container adaptor (public member function)
operator= | assigns values to the container adaptor (public member function)
##### Iterators
begincbegin | returns an iterator to the beginning (public member function)
endcend | returns an iterator to the end (public member function)
rbegincrbegin | returns a reverse iterator to the beginning (public member function)
rendcrend | returns a reverse iterator to the end (public member function)
##### Capacity
empty | checks whether the container adaptor is empty (public member function)
size | returns the number of elements (public member function)
max_size | returns the maximum possible number of elements (public member function)
##### Modifiers
emplace | constructs element in-place (public member function)
emplace_hint | constructs elements in-place using a hint (public member function)
insert | inserts elements (public member function)
insert_range | inserts a range of elements (public member function)
extract | extracts the underlying containers (public member function)
replace | replaces the underlying containers (public member function)
erase | erases elements (public member function)
swap | swaps the contents (public member function)
clear | clears the contents (public member function)
##### Lookup
find | finds element with specific key (public member function)
count | returns the number of elements matching specific key (public member function)
contains | checks if the container contains element with specific key (public member function)
lower_bound | returns an iterator to the first element not less than the given key (public member function)
upper_bound | returns an iterator to the first element greater than the given key (public member function)
equal_range | returns range of elements matching a specific key (public member function)
##### Observers
key_comp | returns the function that compares keys (public member function)
value_comp | returns the function that compares keys in objects of type `value_type` (public member function)
keys | direct access to the underlying keys container (public member function)
values | direct access to the underlying values container (public member function)

### Non-member functions

operator==operator<=> (C++23) | lexicographically compares the values of two `flat_multimap`s (function template)
std::swap(std::flat_multimap) (C++23) | specializes the std::swap algorithm (function template)
erase_if(std::flat_multimap) (C++23) | erases all elements satisfying specific criteria (function template)

### Helper classes

std::uses_allocator<std::flat_multimap> (C++23) | specializes the std::uses_allocator type trait (class template specialization)

### Tags

sorted_equivalentsorted_equivalent_t (C++23) | indicates that elements of a range are sorted (uniqueness is not required) (tag)

### Deduction guides

### Notes

The member types `iterator` and `const_iterator` may be aliases to the same type. This means defining a pair of function overloads using the two types as parameter types may violate the One Definition Rule. Since `iterator` is convertible to `const_iterator`, a single function with a `const_iterator` as parameter type will work instead.

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_flat_map` | `202207L` | (C++23) | std::flat_map and `std::flat_multimap`

### Example

| This section is incomplete Reason: no example

### See also

flat_map (C++23) | adapts two containers to provide a collection of key-value pairs, sorted by unique keys (class template)
multimap | collection of key-value pairs, sorted by keys (class template)
unordered_multimap (C++11) | collection of key-value pairs, hashed by keys (class template)

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